The present invention relates to correction coils employable in nuclear magnetic resonance (NMR) imaging systems. More particularly, the present invention relates to a coil assembly including axial and transverse magnetic field correction coils which are configured in an assembly which prevents relative motion between the coils even at cryogenic temperatures while allowing accurate positioning of the coils as a unit.
In NMR imaging systems, particularly those employed for medical diagnostic purposes, it is necessary to provide a highly uniform and high strength magnetic field. Superconducting magnet coils provide a desirable method for achieving such a field. Superconducting magnets offer a particular advantage in that once energized, no electrical power is needed to maintain the resulting magnetic field. However, NMR imaging imposes strict requirements upon the magnetic field uniformity. In order to reduce the presence of image artifacts, magnetic fields exhibiting spatial variations of only a few parts per million are desired. However, even slight manufacturing variations in the construction of the main magnet can adversely effect magnetic field uniformity. Accordingly, correction coils are generally required to provide corrective magnetic field components. Typically the correction coils carry much less current than do the main magnet coils. Adjustments to the main field provided by correction coils are typically achieved by selecting appropriate current levels and current polarities for the correction coils. In general, correction coils comprise coils or coil sets which are either axisymmetric or axiperiodic. Axisymmetric correction coils typically comprise coil loops which completely surround a cylindrical support form in the circumferential direction. These coils are particularly desirable in adjusting certain axial gradients of the magnetic field. On the other hand, axiperiodic coils are typically configured as arcuate segments joined by axial segments and are commonly referred to as saddle coils. These coils are provided to correct other axial components of the magnetic field within the cylindrical volume. However, of course, the spatial distribution associated with the axial coil corrections and those associated with the saddle coil corrections are significantly different.
Since axial correction coils completely surround the cylindrical coil form upon which they are disposed, it is not possible to employ coil forms with axial slots which extend completely through the form. However, appropriate choices for transverse or saddle shaped correction coils are seen herein as being capable of providing an opportunity for constructing coils with slotted forms.
The reasons which motivate the use of superconductive main magnet coils also provide motivation for the construction of superconductive correction coils. While it is be desirable to include axial and transverse correction coils on a single form, the wiring patterns are generally too complicated to manufacture on a single form to the precision required. Accordingly, multiple forms are employed. However, it must be borne in mind that the electrical conductors preferably comprise superconductive material and accordingly should be cooled to cryogenic temperatures during operation. Therefore, it is preferable to have all of the windings in as close contact as possible with cryogenic coolants such as liquid helium. Such assemblies are operated at temperatures of approximately 4.2.degree. K. The intimate cryogenic fluid contact with the coils is advantageous in preventing and/or minimizing the effects of quench conditions. However, as indicated above, there are stringent requirements with respect to manufacturing tolerances. The correction coils must not only be disposed on their respective forms in fixed positions which prevent conductor motion, but the assembly of distinct coil forms must also be held together in a fashion which prevents relative motion between inner, middle and outer coil forms. The correction coil sets must therefore be accurately positioned with respect to one another and also with respect to the main coils themselves. This insures that the geometric center for each coil set is known. This is a necessity for field corrections. Furthermore, in the case of multiple coil forms, the assembly must be configured so that the outer forms do not lock against one another thereby permitting motion of the inner coil form. It is therefore seen that it is necessary to simultaneously achieve intimate contact between the correction coil conductors and the coolant fluid while at the same time insuring that relative motion between the coils forms is prevented. It must be further borne in mind that these requirements must be met for a coil assembly which is manufactured under room temperature conditions but which is to be immersed in a cryogenic fluid.